The human intervertebral disc is an oval to kidney bean-shaped structure of variable size depending on its location in the spine. The outer portion of the disc is comprised of a tissue known as the annulus fibrosus (or anulus fibrosus, annulus fibrosis, anulus fibrosis, or simply “the annulus”). The inner portion of the disc is comprised of a tissue known as the nucleus pulposus, or simply “the nucleus”.
The annulus is made up of ten to twenty collagen fiber lamellae. The collagen fibers within a given lamella are parallel to one another. Successive lamellae are oriented in alternating directions. About 48 percent of the lamellae are incomplete, but this value varies with location and it increases with age. On average, the lamellae lie at an angle of about 60 degrees to the vertebral axis line, but this too varies with location. The orientations of the lamellae serve to control vertebral motion (i.e., one half of the bands tighten to check motion when the vertebra above or below the disc are turned in either direction).
The annulus contains the nucleus, which has a consistency generally similar to that of crabmeat. The nucleus serves to transmit and dampen axial loads. A high water content (approximately 70-80 percent) assists the nucleus in this function. The water content has a diurnal variation. The nucleus imbibes water while a person lies recumbent. Nuclear material removed from the body and placed into water will imbibe water, swelling to several times its normal size. Activity generates increased axial loads, which squeeze fluid from the disc. The nucleus comprises roughly 50 percent of the entire disc. The nucleus contains cells (chondrocytes and fibrocytes) and proteoglycans (chondroitin sulfate and keratin sulfate). The cell density in the nucleus is on the order of 4,000 cells per microliter.
The intervertebral disc changes, or “degenerates”, with age. As a person ages, the water content of the disc falls from approximately 85 percent at birth to approximately 70 percent in the elderly. The ratio of chondroitin sulfate to keratin sulfate decreases with age, while the ratio of chondroitin 6 sulfate to chondroitin 4 sulfate increases with age. The distinction between the annulus and the nucleus decreases with age. Generally, disc degeneration is painless.
Premature or accelerated disc degeneration is known as degenerative disc disease. A large portion of patients suffering from chronic lower back pain are thought to have this condition. As the disc degenerates, the nucleus and annulus functions are compromised. The nucleus becomes thinner and less able to handle compressive loads. The annulus fibers become redundant as the nucleus shrinks. The redundant annular fibers are less effective in controlling vertebral motion. This disc pathology can result in: (1) bulging of the annulus into the spinal cord or nerves; (2) narrowing of the space between the vertebrae where the nerves exit; (3) tears of the annulus (both “full-thickness” and “partial-thickness” tears) as abnormal loads are transmitted to the annulus and the annulus is subjected to excessive motion between vertebrae; and (4) disc herniation or extrusion of the nucleus through complete (i.e., full-thickness) annular tears. Degenerative disc disease is frequently the cause of substantial pain for a patient.
Current surgical treatments for disc degeneration are generally “destructive”, in the sense that they involve the removal or destniction of disc tissue. One group of procedures, which includes lumbar discectomy, removes the nucleus or a portion of the nucleus. A second group of procedures destroys nuclear material. This group includes chymopapin (an enzyme) injection, laser discectomy, and thermal therapy (i.e., heat treatment to denature proteins in the nucleus). The first two groups of procedures compromise the nucleus of the treated disc. A third group of procedures, which includes spinal fusion procedures, either removes the disc or eliminates the disc's function by connecting together two or more vertebrae, e.g., by “fusing” the vertebrae together with bone. However, such spinal fusion procedures transmit additional stress to the adjacent discs, which typically results in premature degeneration of the adjacent discs. In general, the “destructive” nature of current surgical treatments for disc degeneration can provide substantial pain relief for the patient, but it can also lead to the acceleration of adjacent disc degeneration, which can result in new pain for the patient.
Prosthetic disc replacement offers many advantages. The prosthetic disc attempts to eliminate a patient's pain while preserving the disc's function. Current prosthetic disc implants either replace the nucleus or replace both the nucleus and the annulus. Both types of implants require the removal of the degenerated disc component to allow room for the replacement prosthetic component. Although the use of resilient materials has been proposed, the need remains for further improvements in the way in which prosthetic components are incorporated into the disc space to ensure strength and longevity. Such improvements are necessary, since the prosthesis may be subjected to 100,000,000 compression cycles over the life of the implant.
Current nucleus replacements may cause lower back pain if too much pressure is applied to the annulus. As discussed in U.S. Pat. Nos. 6,878,167 and 7,201,774, the content of each being expressly incorporated herein by reference in their entirety, the posterior portion of the annulus has abundant pain fibers.
Herniated nucleus pulposus occurs from tears (or “fissures”) in the annulus. The herniated nucleus material often applies pressure to the nerves or spinal cord. Compressed nerves can cause back and leg or arm pain. Although a patient's symptoms result primarily from the pressure caused by the herniated nucleus, the primary pathology lies in the torn annulus.
Surgery for the herniated nucleus, which is sometimes referred to as a microlumbar discectomy, only addresses the herniated nucleus. With such surgery, the surgeon removes the herniated nucleus material, which is pressing on the nerves or spinal cord. In addition, in order to reduce the risk of extruding additional pieces of nucleus through the defect in the annulus, the surgeon also generally remove generous amounts of the nucleus still within the annulus. However, this generally requires that the tear or fissure in the annulus be enlarged so as to allow the surgeon access to the nucleus material still within the annulus, and this enlargement of the tear or fissure further weakens the annulus. As a result, while a microlumbar discectomy frequently decreases or eliminates a patient's back and leg or arm pain, the procedure typically further damages the already-weakened discs, which may lead to the creation of future pain for the patient.
Thus there is a need for a new and improved method and apparatus for closing fissures in the annulus.